All fiber lines comprise some type of washing equipment to separate the digestion liquor from the pulp. Later on in the process a washing arrangement is provided to separate bleaching liquors, after the bleaching stages. There exist a number of different types of washing equipment which operate according to different principles.
One type of washing arrangement is the drum washer, where the pulp is dewatered on a rotary filter drum after the addition of washing liquid, which displaces the liquor remaining on the pulp web after preceding process stages, for example a digestion stage or a bleaching stage. The static overpressure causes the displaced liquid to pass through a perforated metal sheet located on the rotary drum. A further development of the original drum washer is the pressurized displacement washer, where the filtrate at overpressure, is caused to pass through the metal sheet. The increase in the pressure difference brings about an improved dewatering of the pulp. The increased pressure difference in the pressurized displacement washer can have the effect that the pulp web deposits itself harder on the metal sheet of the drum, and at times therefore has to be removed by some kind of auxiliary means. The pulp web, for example, can then be loosened by means of liquid or air.
According to a known design of a pressurized displacement washer, the drum is provided with compartments, in which the pulp places itself as oval rectangles in the axial direction of the drum on the metal sheet. Compartmentalization of the drum ensures that the pulp cake does not break up and move, but instead maintains the form produced during application of the pulp. The compartments consist of bars arranged axially along the entire drum, which bars constitute the walls of these compartments. The perforated metal sheet, on which the pulp deposits, is located at a distance from the axle of the drum, so that filtrate channels are formed in the space between the drum and the metal sheet. Along the circumference of the drum there are, thus, at least as many filtrate compartments as pulp compartments. In a drum washer a plurality of different washing stages can be carried out, with separate addition of washing liquid to the different stages, and also with recycling of filtrate from one stage as washing liquid to another stage.
In order to maintain maximum washing effectiveness, it is desired to ensure that washing liquid intended for a specific washing stage is not moved to a later washing stage. Washing liquid intended for a washing stage later in the process is cleaner than the washing liquid used in a preceding washing stage. Due to a pressure difference between the stages the supplied washing liquid tends to move to the lower pressure. In order to be able to separate different washing stages and forming stages, as well as discharge stages, the respective stages are sealed by axial seals, which are placed between the compartment walls of the rotary drum and a pressure-bearing casing extending all about the drum. The filtrates from the respective stages are separated by seals in a peripheral end valve located at one or both end walls of the drum.
In a washing arrangement according to the prior art, the axial seals are positioned in a direction to the center of the drum by means of a number of cylinders, usually 2 or 3 cylinders, which are placed along the length of the seal. The cylinders are controlled in parallel with each other, so that the seal occupies the same position in the radial direction in the entire longitudinal extension of the seal. If the seal becomes worn to a greater extent on one portion, as seen in its longitudinal extension, it would not be possible to remedy this, because the seal switches off by means of spring washers or the like during undesired contact between the compartment walls of the drum and the seal. It is thus sufficient that one portion of the seal is in contact with the compartment wall of the drum for the seal to switch off. If the seal becomes worn unevenly, there is an increased risk of leakage, because the portion most worn will not seal as desired.
One of the objects of the present invention is to solve the aforesaid problems.